Police Supt. Garry McCarthy said Sunday that a group of out-of-town protesters attacked a police van while hundreds of protesters were wandering around downtown Chicago for about 10 hours.

Around 9:30 p.m., while protesters were marching along Jackson Boulevard in the Loop, a police van was trying to push its way through the crowd. Several protesters standing in front of the van tried to push it back, but the van continued going and drove several feet as a number of protesters clung to the front of the vehicle.

McCarthy said Sunday the incident started when several protesters from New York attacked the van, which had only one officer in the cab. McCarthy said that officer suffered a concussion after he was punched through the window. The tires on the van were also slashed, and McCarthy said the officer drove the truck through the crowd to get to safety.